Process for the production of chemically pure phosphoric acid



Patented Aug. 23, 1932 UNITED STATES PATENT OFFICE WILHELM M'O'LLER ANI) JOSEF MARTIN MICHEL, OF BITTERFELD, GERMANY, AS- SIGNORS TO I. G. FARBENINDUSTRIE .AKTIEENGESELLSCHAFT, OF FRANKFORT-ON- THE-MAIN, GER-MANY, A. CORPORATION OF Y PROCESS FOR THE PRODUCTION OF GHEMICALLYjPURE PHOSPHORIC ACID No Drawing. Application filed September 19, 1929, Serial No. 393,837, and in Germany September 25, 1928.

This invention relates to the production of chemically pure phosphoric acid.

For purifying acids, it has been proposed to subject them to electrolysis (see Lunges Schwefelsaurefabrikation, 2nd Volume, pages 1198/1199), but no particulars were a successful refinement of acids.

For purifying technical phosphoric acid which, as a rule, contains arsenic, arseneous acid, phosphorous acid and heavy metals as the most undesired contaminations, various purely chemical methods have been proposed, which, however, generally contemplate the removal of only a single one of these .contaminations, in the most cases arsenic.

Now we have found that all the abovementioned contaminations can be removed from the raw acid in a single operation and in a very simple manner by subjecting the raw acid to an electrolytic refinement, whenemploying cathodes upon which the metals or metalloids obtained from the contaminations by cathodic reduction are deposited without simultaneously reducing the orthophosphoric acid to lower phosphoric oxides. Chromium, platinum or tantalum .may be employed for the cathodes, the best results,

. however, bein obtained with copper or silver 'lll cathodes, as these require the lowest decomposition voltage whereby a maximum output of energy is attained. For the anodes only platinum practically comes into consideration. Preferably the cathodic current density is kept below 0.01 amperes per square centimeter, for instance, at 0.005 amperes per phorous acid are oxidized at the anode to orthophosphoric acid, whereas all metallic contaminat1ons, as well as arsenic and antimony are separated at the cathode. When the electrolysis is completed, the acid is filtered so as to separate particles of the slime I precipitated at the cathode which may have passed over into the acid. Moreover our improved process aifords the remarkable adinstance, an acid of 80 to 90%, can be refined immediately after its fabrication. The refining methods hitherto used are limited to acid concentrations of 25% asan upper limit, given as to the essential details required for as the usual precipitation of the heavy metals in the form of sulfides by means of sulfurettedhydrogen is only possible with such low concentrations. In order to oxidize the phosphorous acid when working according to this old method, nitric acid must'be added after-filtration, and finally the solution is evaporated to the desired concentration in quartz vessels.

Ea /ample 1 Milligrams per liter Phosphorous acid 490 Arsenic 25 Heavy metals, including iron 240 The raw acid was electrolyzed with anodesof platinum and cathodes of copper, the .cathodic current density amounting to 0.004 amperes/cm? and the anodic current density to 0.25 amperes/cm After 4 to 5 kiloampere-hours had passed through, the acid proved to be free from the above-mentioned contaminations and met all requirements demended of a pharmaceutically pure acid.

Further experiments have shown that electrolytic purification of the acid may be ef-' fected more rapidly and with better output of energy, when working at elevated temperatures of the electrolytic bath, viz. atabout 50 C. or more. The heating may be reflected either by the electrolyzing electric current itself, or by external means. However, the former method is preferred in most cases, as it may be carried out in a very simple manner. We have further observed that in some cases, when either none, or only an extremely small quantity of heavy metals are present in the raw acid, the electrolytic separation-of arsenic is not easily attained, but may be promoted by the presence of relatively small qllllantities of copper salt, probably owing to t e fact that a copper arsenide compound is deposited upon the cathode. Therefore small quantities of a copper salt, preferably copper phosphate, are added to the raw acid, to facilitate the separation of the arsenic in absence of heavy metals. The raw acid to be refined is analyzed and the copper salt added to such an extent that the ratio of .Cu-to As is brought to 1 to at least 2, preferably 1 t0 2.5-4. 7

Example 2 A raw acid showing a specific gravity of 1.70 at 17.5 C., correspondin to a concentration of 84.5% H PO was su jected to our improved process. This raw acid contained the following contaminations Milligrams per liter Phosphorous acid 520 Arsenic 115 Heavy metals, except iron Iron- 20 The raw acid was electrolized with platinum anodes and copper cathodes at a bath temperature of about C. with the addition of 0.2 grams of copper phosphate Gu (P00 .3 H O, per liter, the cathodic current density being kept at 0.004 amp/cm and the anodic current density at 0.25 amp/cm After 3 kiloainpere-hours had been passed through, the acid was found to be ,free from contaminations and answered the requirements of the Pharmacopoea Germanica, 6th edition. The elevation of the temperature up to 50 C.-was attained by simply embaling the electrolyzers. The voltage was thereby reduced from 22.5 to 18' volts, corresponding to a saving of energy of about 20%.

The technical progress of our improved process is due to its extraordinary simplicity as compared with the methods hitherto used, as acids of any concentration can be subjected to the refinement according to the present invention and the refined products are immediately ready for further use. This progress was rendered possible by recognizing that for the electrolytic refinement of phosphoric acid it does not suflice to subject the phosphoric acid to the action of a direct current between and acid-proof electrodes,but that the maximum current density of about 1 ampere per square decimeter must be employed in order to avoid the reduction of phosphoric acid, whereas the anodic current density may be higher.

We claim: I 1. The process of producing chemically pure phosphoric acid from a technical raw acid .which comprises: subjecting the raw phosphoric acid to electrolysis, cathodes being used upon. which'the contaminations are deposited in the'form of metals and metals loids without the phosphoric acid being reduced to lower stages of oxidation, the cathodic current density not exceeding 1 ampere per square decimeter.-

2. The process of producing chemically pure phosphoric acid from a technical raw acid which comprises subjecting the raw phosphoric acid to electrolysis at a bath temperature exceeding 50 (3., cathodes being used upon which the contaminations are deposited in the form of metals and metalloids without the phosphoric acid being reduced to lower stages of oxidation, the cathodic current density not exceeding 1 ampere per square decimeter.

\ 3. The process of producing chemically pure phosphoric acid from a technical rawacid poor in heavy metals which comprises adding a small quantity of a cop er salt to the raw phosphoric acid and sub ecting the raw acid to electrolysis, cathodes being used upon which contaminations are deposited in the form of metals and metalloids without the phos horic acid being reduced to lower stages 0 oxidation, the cathodes current density not exceeding 1' ampere per square decimeter.

4. The process of producing chemically pure phosphoric acid from a technical rawacid poor in heavy metals which comprises acid poor in heavy metals which comprises adding a copper salt to the raw phosphoricv acid to such an extent that the ratio of Cu to As is confined between the limits 1:2 and 1: 4, and subjecting the raw acid to electrolysis, cathodes being used upon which the contaminations are'de osited in the form of -metals and metalloi s without the phos-.

phoric acid being reduced to lower stages of oxidation, the cathodic current density not exceeding 1 ampere per square decimeter.

In testimony whereof, we aflix our signs.

tures.

WILHELM MULLER. JOSEF MARTIN MICHEL. 

